This invention relates to optical waveguides and more particularly, to a multiple core optical fiber for use in a secure communication system.
The increased burden on communication systems has fostered the development of high capacity systems using optical waveguides. These optical waveguides are constructed of a transparent dielectric material such as glass. They consist of a central core surrounded by a cladding having an index of refraction less than that of the core. Light propagates along the waveguide. The theory of optical waveguides is contained in U.S. Pat. No. 3,157,726--Hicks et al and in a publication "Cylindrical Dielectric Waveguide Mode", by E. Snitzer, Journal of the Optical Society of America, Vol. 51, No. 5, pp. 491-498, May 1961.
Recently, however, optical waveguides having very low attenuation per unit length have been developed. For example, the Maurer and Schultz, U.S. Pat. No. 3,659,915, "Glass Optical Waveguide", describes an optical waveguide comprising a cladding layer of pure fused silica or doped fused silica and a core formed of doped fused silica. Germania and silica glass waveguides can also be used in accordance with U.S. Pat. No. 3,884,550. Single mode waveguides fabricated in accordance with Keck and Schultz, U.S. Pat. No. 3,711,262, are also suitable for use.
Optical waveguides having multiple cores are also known, one example being shown in U.S. Pat. No. 3,930,714.
In many communication systems, it is essential that the sender be able to recognize when his communications have been compromised. Normally, quiescent emanations from an optical waveguide are not detectable by an intruder. It is necessary for the intruder to perturb the waveguide, for example bend it, to enhance emanations from the waveguide so that the signal can be detected.